The merits of wall cavity thermal insulation

HomeArticlesThe merits of wall cavity thermal insulation
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If you are involved in the building sector in any way, be it as specifier, contractor, developer or new build customer, you will have been exposed by now to the regulatory requirement to ensure the roof of buildings provides a certain thermal protection to resist heat flows inwards in summer and outwards in winter. Given our modern building practices, this invariably means adding substantial amounts of insulation to the roofing system.

Our country’s need to manage energy usage, as well as control emissions in terms of international protocols to which we are signatories has led to the 2011 amendment to the National Building Regulations SANS 10400, section XA: Energy Usage in Buildings, as well as SANS 204:2011 Energy Efficiency in Buildings.

The roof is considered the main source of heat gain or loss, and is specifically addressed in the regulations in terms of interventions required to reduce energy usage. Floors are only regulated and insulated where underfloor heating is planned.

Low mass external walls such as timber or steel framed structures have stipulated minimum thermal resistance R values to achieve, dependant on climatic zone.

Masonry walls generally meet the requirements of the regulations, and require no stipulated interventions such as insulation. However, there is a case to be made for including insulation in the wall cavity, even in a country with relatively moderate temperatures, such as South Africa.

The first consideration is that as the footprint size of houses becomes smaller, the ratio of roof to wall area decreases, making the walls more relevant in terms of heat flow in and out. Also, when houses get taller through the addition of extra floors, the wall area can far exceed the roof area. As houses rise taller above shading trees and shrubbery, walls are more susceptible to direct heat impingement, particularly on the east and west sides.

Cavity wall insulation implies the inner masonry leaf is at internal temperature, rather than external temperature. The insulation increases the effect of the thermal mass of the bricks, causing the internal temperature to remain constant for longer. If you are heating or cooling within the home, the insulated walls help to maintain your desired temperature, and reduce the cost of the heating or cooling to achieve your comfortable home.

Good thermal insulation also acts as a moisture barrier, preventing moisture transferring through the inner leaf. As the walls are warmer, condensation on the internal wall surface is eliminated.

Cavity wall insulation comes into its own when a competent person performs a rational design to minimise the energy demand and usage of a particular building. Using cavity wall insulation will reduce the heat flows sufficiently to allow the designer to perhaps reduce roof insulation and high spec glazing interventions, which would otherwise be required if using the prescriptive route to satisfy the requirements of SANS 10400 XA. Beyond enabling considerable cost savings, performing a rational design allows the architect and client the freedom to have the design they want which maximises the benefits of the site, yet still minimising energy demand and usage.

IsoBoard XPS is the ideal insulator for use within cavity walls, given the highest thermal and moisture resistance, and particularly it’s durability. IsoBoard will perform effectively for the life of the building, an important consideration when installing Insulation that can never be replaced.

Please see our website for technical installation details and our Agre’ment certificate for this application, or talk us for advice on the cavity wall insulation application.